Researchers have discovered that a virus commonly found in dogs may lead to a breakthrough in human vaccine development.

Parainfluenza virus 5, or PIV5, is harmless to humans but is thought to contribute to upper respiratory infections in dogs, and is a common target for canine vaccines designed to prevent kennel cough. In a new paper, researchers describe how this virus could be used in humans to protect against diseases that have eluded vaccine efforts so far.

Because PIV5 does not cause disease in humans but our immune system recognizes and destroys it, antigens from other viruses or parasites effectively use PIV5 as a delivery vehicle that exposes the human immune system to important pathogens and allows it to create the antibodies that will protect against future infection, an approach which not only ensures full exposure to the vaccine but also is much safer because it does not require the use of attenuated, or weakened, pathogens. For example, an HIV vaccine delivered by PIV5 would contain only those parts of the HIV virus necessary to create immunity, making it impossible to contract the disease from the vaccine.

"We can use this virus as a vector for all kinds of pathogens that are difficult to vaccinate against," said Biao He, the study's principal investigator and professor of infectious diseases in UGA's College of Veterinary Medicine. "We have developed a very strong H5N1 flu vaccine with this technique, but we are also working on vaccines for HIV, tuberculosis and malaria. Safety is always our number one concern. PIV5 makes it much easier to vaccinate without having to use live pathogens."

Using viruses as a delivery mechanism for vaccines is not a new idea, but in practice it is inconsistent. If humans or animals already possess a strong immunity to the virus used for delivery, the vaccine is unlikely to work because it will be destroyed by the immune system too quickly. Pre-existing immunity to viruses is the main reason most of those vaccines fail.

But He and colleagues demonstrate that immunity to PIV5 does not limit its effectiveness as a vaccine delivery mechanism, even though many animals, including humans, already carry antibodies against it.

In their experiments, the researchers found that a single dose inoculation using PIV5 protected mice from the influenza strain that causes seasonal flu. Another single dose experimental vaccine also protected mice from the highly pathogenic and deadly H5N1 virus commonly known as bird flu.

"I believe we have the best H5N1 vaccine candidate in existence," He said. "But we have also opened up a big field for a host of new vaccines."